Bag filters have been used in the past for filtering fluids (see, for instance, U.S. Pat. Nos. 5,358,638, 5,417,855 and 5,755,962). Conventional bag filters, however, have a relatively limited area of flow and filtration and hence have a limited operating life.
Attempts have been made to provide filters with an enhanced operating life and efficiency. For instance, U.S. Pat. Nos. 5,358,638 and 5,755,962 disclose filter bag subassemblies having a plurality of filtering layers, while U.S. Pat. No. 4,877,526 discloses a flexible filter bag having a strip of flexible filtering fabric wound itself in a large number of convolutions. Moreover, U.S. Patent Publication No. U.S. 2002/0027100 relates to a filter element assembly having at least two cylindrically shaped and concentrically arranged filter media sleeves, while a pleated filter cartridge sold by Rosedale Products, Inc. has rigid pleating for filtering fluids (see, also, U.S. Pat. Nos. 5,036,321, 5,552,048, 5,762,792 and 5,988,400 for pleated filter cartridges). In addition, U.S. Pat. No. 5,549,826 discloses a filtration bag having an outer fabric sleeve and an inner fabric sleeve, which has a pleated configuration. It is, however, believed that the foregoing filter devices have various shortcomings and disadvantages (e.g., they do not provide a sufficiently large filtering surface and/or are relatively expensive to manufacture and/or use).